1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the same, more particularly to a method for forming metallic wirings of a semiconductor device through polishing and a semiconductor device manufactured using the method.
2. Description of the Related Art
In recent years, planarization of the surfaces of wiring boards has been increasingly valued for large scale semiconductor integrated circuits (LSIs). One of the representative techniques for such the planarization is the Chemical Mechanical Polishing (CMP: hereafter, to be described as polishing otherwise specially defined), which is disclosed in, for example, the U.S. Pat. No. 4,944,836. According to this method, for example, a metal film for wiring is formed on a silicon substrate (wiring board) on which an LSI is to be formed, then the metal film is treated through well-known lithography and Reactive Ion Etching (RIE) techniques, thereby forming the object wiring pattern. After that, an insulating layer is formed on the wiring pattern. If the surface of the insulating layer is planarized through polishing, the fabrication accuracy of the wiring to be formed in the upper layer will be improved more effectively.
On the other hand, the damascene method that forms metallic wirings through polishing steps is also a focus of attention. According to the damascene method, an insulating film is formed on a wiring board at first, then grooves are formed in the insulating film for the wiring using both well-known lithography and RIE techniques. On the board with those grooves is then deposited a metal film for wiring. After that, the metal film except for the portion inlaid in each groove is removed through polishing, so that inlaid metal lines are formed there. This technique is disclosed in, for example, Japanese Patent Laid-Open No. 2-278822. This method, which is effective especially when lines are to be formed with a copper based alloy whose fine fabrication by RIE is difficult, is also now under study as a method for fine fabrication of aluminum wiring patterns.
The details of the conventional damascene method are introduced in the official gazette of Japanese Patent Laid-Open No. 10-214834 and in the publication of Proceedings CMP-MIC Conference (pp.415-422). Those examples describe a layered film consisting of tungsten and titanium respectively. Hereunder, the damascene method will be described on the basis of those examples with reference to FIG. 3. At first, an insulating layer 31 is formed on a wiring board 30 as shown in FIG. 3(a). Then, in the insulating layer 31 are formed depressions such as grooves or holes (to be referred to as grooves 32 generically) to be used as wirings and interlevel portions. After that, a lower metallic layer 32 and an upper metallic layer 33 are formed in order so as to cover the insulating layer 31. The lower metallic layer 32 consists of titanium and the lower metallic layer 33 consists of tungsten.
After that, both upper and lower metallic layers 32 and 33 except for the grooves are removed through polishing as shown in FIG. 3(b). Since the upper metallic layer 33 to be polished in the dense area of each groove pattern is substantially thinner than that of other areas provided with no pattern, the polishing of the upper metallic layer 33 is ended earlier in the pattern dense area than in other non-pattern areas. Therefore, when the lower metallic layer 32 and the first insulating layer 31 are exposed, part 33a of the upper metallic layer 33 still remains unfinished in each no-pattern area. And, if the polishing is continued until the lower metallic layer 32 is removed completely, phenomena referred to as erosion and dishing occur. The erosion means a phenomenon that the surface of the insulating layer 31 is depressed by a depth E1 more in each pattern dense area than in other non-pattern areas as shown in Fig. (c). The dishing means a phenomenon that the surface of the upper metallic layer 33 is depressed by a depth D1 more in each groove from the surface of the surrounding insulating layer 31. It is also reported that a localized erosion occurs at a depth of LE especially at each boundary between a pattern dense area and a no-pattern area. It is also reported that this localized erosion depends on the relative moving directions of both polishing platen and wiring board, as well as the type of the polishing pad, etc.
The reason why such a height difference occurs is explained as follows. Generally, a polishing agent for the layered metals causes the polishing rate to be reduced in order of upper metallic layer, lower metallic layer, and insulating layer. In particular, a significant difference appears between polishing rates for the lower metallic layer and the insulating layer. In most cases, the polishing rate of the insulating layer is only a factor of scores with respect to the polishing rate for the lower metallic layer. Since the polishing rates for the upper and lower metallic layers 32 and 33 are larger than that of the insulating layer such way, the surfaces of the upper and lower metallic layers 32 and 33 in each pattern dense area are depressed deeply from the surface for the insulating layer 31 when the polishing ends. Consequently, a large polishing force is concentrated effectively at each projection of the insulating layer 31.
The polishing of the insulating layer is thus accelerated in each pattern dense area according to this difference between such the polishing forces, thereby causing a large height difference. And, a total of such an erosion depth E1, a localized erosion depth LE, and a dishing depth D1 would often reach xc2xd of the initial depth of the groove. If an inlaid metal is used for wiring, therefore, such a depth will increase the resistance of the object wiring significantly. In addition, if an attempt is made to form a multi-layered wiring on this layer using the damascene method, the height differences generated by such the erosion depth E1, the localized erosion depth LE, and the dishing depth D1 will affect the wiring in the upper layer, causing the metallic layer to be left unpolished in each depression. And, this unpolished portion of the metallic layer causes short-circuits of the wiring in the upper layer.
There is a two-step polishing method proposed for minimizing the difference between polishing rates effectively by eliminating height differences as described above. Concretely, the polishing is finished once after the polishing of the upper metallic layer 33 is almost finished as shown in FIG. 3(d). Since the polishing rate for tungsten is five times faster than that for titanium, the polishing can be finished before the titanium film is removed completely. Next, the polishing agent is changed to another that contains silica abrasive powder and potassium hydroxide or ammonium hydroxide, thereby polishing the titanium of the lower metallic layer 32. The use of this polishing agent can almost equalize the polishing rate between the insulating film 31 and titanium. The polishing method will thus reduce the erosion depth E2 and the dishing depth D2 as shown in FIG. 3(d), thereby improving the planarization of the surface of the layer. This conventional two-step polishing method aims at elimination of polishing non-uniformity caused by the polishing of an upper or middle layer (titanium containing nitrogen, if necessary) by stopping the polishing (the first polishing) on the surface of the titanium film, which is a lower metallic layer. The first polishing is done for layers including a thick tungsten film, which is an upper metallic layer, as well as a middle titanium nitride layer. The conventional two-step polishing method further aims at actual reduction of the non-uniformity of polishing only in the second polishing of the lower metallic layer. In the second polishing, the polishing rates for both lower metallic layer and insulating layer should preferably be equal or over. Actually, however, the polishing rate for the insulating layer is faster than or almost equal to that for the lower metallic layer. The conventional two-step polishing method also aims at reduction of the dependency of the polishing rate on pattern by equalizing both polishing rates. According to this polishing method, therefore, the planarization of the surface of the insulating layer is improved after the polishing. More concretely, the polishing of the insulating layer is increased significantly. Consequently, the residual insulating film becomes thinner contrarily in some cases. However, since such a structure is used mainly as plugs for interlevel connection conventionally after tungsten is inlaid in the film, it has been considered that a metallic layer should rather be protruded from the insulating layer surface as disclosed in the official gazette of Japanese Patent Laid-Open No. 9-167768. Such way, the planarization of the surface of the insulating layer has been considered to be important even when the insulating layer is thinned.
The official gazette of Japanese Patent Laid-Open No. 8-83780 discloses a polishing method for copper based alloys. The gazette also describes a polishing method for a copper based alloy film and a tungsten film, as well as favorable characteristics of a polishing agent, which are obtained when the polishing agent contains alumina abrasive powder, a metallic layer oxidizer, and etching chemicals. Concretely, while the surface of an object metallic layer is oxidized using an oxidizer, the oxide is removed mechanically by abrasion due to the friction with inorganic abrasive powder and polishing pad. This is accepted as a basic polishing mechanism. The etching chemical is used to increase the polishing of metallic layers. If such the etching chemical is added to a polishing agent, however, etching also proceeds onto the metallic layer in each groove which comes in contact with neither abrasive powder nor polishing pad, thereby the planarization of the surface is degraded. In order to avoid such a problem, corrosion-proof agent, i.e., inhibitors are added to the polishing agent. Dishing can be prevented by increasing the ratio of the polishing rate to the etching rate, thereby improving the fabrication accuracy. In this example, the pH (hydrogen exponent) of the polishing agent is assumed to be neutral. If a corrosion-proof agent is added to the polishing agent, however, the polishing rate is lowered significantly. More concretely, if a corrosion-proof agent is added by 0.1%, the polishing rate is lowered down to 10 nm/min. Consequently, the ratio between polishing and etching rates is not always increased in proportion to the concentration of the corrosion-proof agent. Since the actual polishing rate must be 100 nm/min or so, such the characteristics will not be suitable for practical uses. In order to avoid occurrence of such a dishing phenomenon, there is also proposed a substantially two-step polishing method, which polishes a copper based alloy at the room temperature, then polishes a barrier layer mainly by lowering the temperatures of both polishing platen and polishing agent, thereby changing the polishing characteristics.
The following polishing agents available on the market are well known. QCTT1010 (a product of RODEL Inc., to be mixed with a hydrogen peroxide solution when used) contains alumina abrasive powder to function as a polishing agent for copper based alloys. And, the following are well known as polishing agents for tungsten: SS-W2000 (a product of CABOT Inc., to be mixed with a hydrogen peroxide solution when used) including silica abrasive powder, WA-400 (a product of CABOT Inc., to be mixed with an FE-400 solution including ferric nitride when used) including alumina abrasive powder, MSW-1000 (a product of RODEL Inc., to be mixed with a hydrogen peroxide solution when used), XGB-5542 (a product of RODEL Inc., to be mixed with potassium iodate when used), etc. On the other hand, the polishing agents should preferably be alkaline in order to increase the polishing of the above-mentioned insulating film. Generally, each of those polishing agents contains silicon abrasive powder, ammonium hydroxide, and potassium hydroxide. Polishing of insulating films has been used for a long time and many products have appeared on the market. It is well known that the hydrogen exponent (pH) of the solution is changed depending on the concentration of ammonium hydroxide and potassium hydroxide, thereby the polishing characteristics are changed. In the official gazette of Japanese Patent Laid-Open No. 10-214834, such a type polishing agent is used for polishing a titanium layer.
However, since this two-step polishing method makes the polishing rates for both lower metallic layer 32 and insulating layer 31 closer as described above with reference to FIG. 3, polishing of the insulating layer 31 is speeded up if a polishing agent in use consists of silica abrasive powder and an alkaline solution of potassium hydroxide, ammonium hydroxide, etc. Consequently, while the height difference on the surface of the insulating layer is reduced significantly, the no-pattern insulating layer 30 is also polished away when this method is used. The insulating film is thus thinned by a value xcex94T shown in FIG. 3(e). And, the total of the erosion E2, the dishing depth D2, and the reduction xcex94T in the film thickness will reduce each groove in depth significantly, as well as the inlaid metallic layer in thickness. The use of such an insulating layer for wiring will thus increase the resistance of the wiring. This is why the insulating film will not be satisfactory yet in quality.
The polishing rate for titanium used for the lower metallic layer 32 is actually almost the same as that for the insulating layer 30. The upper limit ratio between the polishing rates for both layers 32 and 30 is concretely described in the 0058 field as 120 nm/min for the polishing rate for titanium and 25 nm/min for the polishing rate for oxide films if potassium iodate is used for the polishing agent.
Consequently, the polishing rate for titanium becomes 4.8 times (120/25=4.8) that for oxide films. This polishing agent can be used as the first polishing agent. The second polishing agent should be another (0060 field), however. It will be understood therefore that the polishing rate of the second polishing agent for the lower metallic layer should preferably be almost equal to that for the insulating layer and must be not more than 4.8 times that for the insulating layer. In spite of this, if polishing is done excessively by ten or scores of % in consideration of the polishing rate distribution, the structure over a plurality of wiring boards, and the polishing rate variability in a large area wiring board, then the resistance of the wiring is varied significantly due to the variability of the grooves in depth in the wiring board, and by extension, due to the difference between the wiring layers in thickness. Thus, polishing selectivity for lower metallic layer, i.e., polishing rate ratio of that for lower metallic layer to that for the underlying insulating layer, of 4.8 or less will not be enough.
In order to form low-resistance wirings in uniform all over the surface of a wiring board, therefore, it is required to remove both upper and lower metallic layers from predetermined portions of the entire surface of the wiring board, for example, from the entire region in which wiring circuit elements are formed, without thinning the insulating layer 31 at all.
Under such the circumstances, it is an object of the present invention to form wirings by suppressing the degradation of their shapes so as to keep both yield and stability of the production satisfactorily when multi-layered damascene wirings are formed by removing the first alloy layer. Each of the damascene wirings consists of an upper metallic layer, and a lower metallic layer which is effective as a barrier layer.
The official gazette of Japanese Patent Laid-Open No. 10-214834 has never described chemical etching in polishing. It is thus unknown how much the etching proceeds chemically in the polishing process.
(1) Firstly, the above object of the present invention is achieved by polishing the lower metallic layer faster than the underlying insulating layer in the second polishing process after the first polishing of the upper metallic layer is finished. Concretely, when the second polishing is done for the lower metallic layer 12 shown in FIG. 1(b) after the first polishing of the upper metallic layer 13 shown in FIG. 1(a), the lower metallic layer 12 is polished faster than the insulating film 11. Actually, the polishing rate for the lower metallic layer should preferably be not less than five times that for the underlying insulating film. In order to improve the polishing accuracy more significantly, the multiple of the polishing selectivity between those layers should preferably be increased up to 7 to 10. Principally, there is no upper limit for this ratio between those polishing rates. For example, if a polishing agent that contains no abrasive powder substantially is used for polishing the lower metallic layer, the selectivity between the polishing rates will exceed 10 easily. If the polishing selectivity of the layer to the underlying insulating film is increased such way, the underlying insulating film will be suppressed from thinning effectively, thereby reducing the wiring resistance. In this specification, a polishing agent including no abrasive powder substantially shall mean a polishing agent containing an amount of abrasive powder of not more than 0.1 wt %.
Hereunder, changes of such a polishing process with time will be described with reference to FIG. 4. In order to simplify the description, it is premised that the subject wiring consists of two layers (upper and lower metallic layers). In the first polishing, the upper metallic layer is polished quickly. Then, if the polishing rate for the lower metallic layer is low as shown in FIG. 4(a), the polishing stops almost on the surface of the lower metallic layer actually. Excess polishing will affect little on the shape of those layers even when it is done. After that, the second polishing is done. The polishing rate for the insulating layer is lower significantly than that for the lower metallic layer. Consequently, the polished thickness is very small for the insulating layer at a portion where the maximum polished thickness appears by the polishing rate deviation even when excessive polishing is further done by more 10% or 20% after the polishing is finished at a portion where the polishing is done at the minimum (Min) rate. The depth variability is reduced in proportion to an increase of the polishing selectivity. If the polishing rates of both lower and upper metallic layers are almost equal, the first polishing is finished almost on the surface of the insulating layer without doing any excess polishing as shown in FIG. 4(b). The first polishing may be finished when the polishing is carried out for the time period estimated to be needed to remove the metallic layers by using average polishing rate. Though the insulating layer will be polished away slightly, the polished thickness is suppressed small almost in inversely proportion to the polishing selectivity of the upper or lower metallic layer to the underlying insulating layer. After that, the second polishing is done thereby to remove the lower metallic layer. Excess polishing may be done by more 10% or 20% after the polishing is finished at the minimum (Min) polishing rate portion. In this case, the deviation of the groove depths will be increased slightly more than in the case shown in FIG. 5(a). However, since the polishing rate for the insulating layer is lower significantly than that for the lower metallic layer in this case, the variability is reduced far more than that of the conventional technique. In the case where the effect of the present invention is very distinct, the depth variability including dishing and erosion can be reduced to ⅓ to ⅕ of the conventional one.
(2) Secondly, the above object of the present invention is achieved by polishing the lower metallic layer (in the second polishing process) faster, concretely five times or more faster than the etching rate for the upper metallic layer, after the first polishing of the upper metallic layer is finished. Conventionally, it has been said that if the polishing selectivity is set so that the polishing rate for the lower metallic rate becomes faster than that for the insulating layer, the surface of the lower metallic layer is depressed, thereby the planarization is degraded on the contrarily. Inventors of the present invention are aware of the point that polishing is done with a combination of a chemical effect and a mechanical abrasive effect. If the chemical effect is added to the mechanical abrasive effect for polishing, therefore, etching will proceed more or less due to a chemical reaction of components in the polishing agent with a metallic layer during the polishing. This etching rate affects the polishing accuracy significantly. If the etching rate for the upper metallic layer is large, the upper metallic layer 33 will be depressed as shown in FIG. 3(e) when the lower metallic layer 32 is polished. As a result, dishing and erosion, which are especially problems in the damascene method, will occur. Concretely, the ratio of the polishing rate for the lower metallic layer to the etching rate for the upper metallic layer should be not less than 5, more preferably not less than 10. In addition, the ratio should preferably be not less than 15 if the wiring layer thickness needs being kept in highly uniform all over the surface of the object wiring board.
(3) Thirdly, the above object of the present invention is achieved by etching the upper metallic layer during the first polishing at not more than ⅕, more preferably at not more than {fraction (1/10)} of the polishing rate for the upper metallic layer, then carrying out the second polishing for the lower metallic layer. Such the polishing method will make it easier to control the polishing of the upper metallic layer, as well as remove the lower metallic layer stably. Of course, in addition to those polishing conditions, the etching rate for the upper metallic layer should be set to not more than ⅕, more preferably at not more than {fraction (1/10)} of the polishing rate for the lower metallic layer.
According to the method described in (2) or (3), the insulating layer is polished away only very slightly during the first or second polishing, since the pH of the polishing agent is acid. In this specification, an area whose pH value is not more than 7.5 and not less than 6.5 shall be referred to as a neutral area and an area whose pH value is below 6.5 is referred to as an acid area. And, an area whose pH is over 7.5 is referred to as an alkaline area.
In order to increase the rate ratio of the polishing rate to the etching rate, the following method is usable. Generally, a polishing rate is increased in proportion to an increase of the polishing force and the rotational speed of the polishing platen in addition to the characteristics of the object polishing agent. On the other hand, an etching rate does not depend so much on the polishing force, etc.; it is decided almost by the characteristics of the object polishing agent. Consequently, the conditions of the polishing force and the rotational speed of the platen should be set large. If a low polishing force is needed, the etching speed should also be low for the object polishing agent.
(4) Fourthly, the above object of the present invention is achieved by providing a semiconductor device, wherein the lower metallic layer inlaid in the groove or via hole of the insulating film is formed so as to become higher than the surface of the upper metallic layer formed in the groove or via hole. Such a structure of the semiconductor device will make it possible to prevent the upper metallic layer from coming in contact directly with the insulating film formed on the side wall, thereby improving the reliability thereof.
FIG. 2(d) shows details of an end of an inlaid wiring pattern formed in accordance with the present invention. The edge of the surface 25 of the insulating film 21, which is in contact with a metallic layer 22 and 23, has a slight tilting portion generated due to a friction with a polishing pad. The tilting angle is not more than 1xc2x0. At the end portions 24 of both upper and lower metallic layers, the polished surface of the lower metallic layer 22 is equal to or slightly lower than the polished surface of the insulating layer 21. The height difference between them is not more than 5 nm, however. The polished surface of the upper metallic layer 23 is also 5 to 10 nm lower than the polished surface of the lower metallic layer 22. This is because the upper metallic layer 23, which consists of copper, is very easy to be etched, thereby the upper metallic layer is also etched slightly when it is polished. However, the tilting angle of the polished surface of the upper metallic layer 23 is not more than 1xc2x0 in the direction it goes away from the lower metallic layer 22; the wiring layer is formed such accurately.
The configuration of the above-described invention is determined by the characteristics of the polishing to be described below.
The present inventor, et al, have found that it is possible to control both polishing rate and etching rate set for an object polishing agent respectively almost independently of each other without changing the polishing temperature including that of the wiring board if the composition of the polishing agent is selected properly. It will thus be understood that it is possible to reduce occurrence of erosion and dishing phenomena, as well as localized depressions (localized erosion) significantly without increasing the polishing rate for the object insulating layer.
Hereunder, description will be made for the findings with respect to why an etching rate set for a polishing agent is important. Generally, an etching rate is measured while the polishing agent solution is static. (This is why this etching rate is also referred to as a static etching rate.) In an actual polishing process, however, the polishing agent is moved fast on the surface of the object wiring board. The moving speed is 10 to sores of meters/min. The inventor of the present invention, et al, have found that the etching rate of a solution, while it is moved fast, becomes several times faster than that in the static state. In addition, if polishing is to be done for a plurality of metallic layers, each etching rate of those metallic layers must be taken into consideration to decide the etching rate. In this case, it is assumed that there are only two metallic layers (upper and lower). The number of metallic layers is varied freely, of course. At first, the first polishing is carried out for the upper metallic layer. When the first polishing is finished, both upper and lower metallic layers appear on the surface of the wiring board. Generally, the first polishing must be done excessively. And, the etching rate for the upper metallic layer must be lower than the polishing rate for the upper metallic layer so as to suppress the dishing to occur in the upper metallic layer during this excessive polishing. The ratio of the polishing rate to the etching rate should be not less than 5, preferably not less than 10.
The etching rate for the lower metallic layer should also be set lower than the polishing rate for the upper metallic layer so as to suppress the dishing of the lower metallic layer. This is not required, however, if there is provided means for forming a protection layer, etc. after the second polishing to be described later even when dishing appears in the lower metallic layer due to etching. In this case, however, the damascene wiring process will become complicated. On the other hand, in the second polishing process, polishing is started while both the upper and the lower metallic layers exist and continued until the lower metallic layer formed on the insulating layer surface is removed. Consequently, the polishing rate for the lower metallic layer must be set faster for the second polishing agent than the etching rate for both upper and lower metallic layers. The ratio of the polishing rate to the etching rate should also be not less than 5, preferably not less than 10. In addition, the ratio should be not less than 15 if the increase of the resistance of the wiring has to be suppressed sufficiently, and the resistance of the wiring must be kept highly in uniform all over the surface of the wiring board having a large area whose diameter is not less than 150 mm.
It was thus found that using such a polishing agent could protect the surface of the object metallic layer from depression negligibly during polishing even when the ratio between the polishing rate selectivity for the metallic layer against for the insulating layer was large. Accordingly, it was also found that occurrence of both erosion and dishing phenomena could be suppressed. Generally, the following ratio is obtained with respect to the erosion etching rate between static and stirring states. For such a metal as a copper based alloy which is chemically reactive, many of conventional polishing agents have strong etching properties. Such a polishing agent with strong etching properties gives a large etching rate in the static state. Usually, the etching rate exceeds 5 nm/min in the static state. In the stirring state, it becomes 5 to 10 times that in the static state. Since the etching action on the surface of a copper based alloy is strong, the etching rate seems to be increased significantly when the polishing agent is stirred so as to keep supplying of a fresh solution. It was thus found in the tests performed by the present inventor, et al., the etching rate in stirred states became two times at highest to that in static state, when the different slurry which has very small static etching rate, for example, not more than 5 nm/min, was examined.
For titanium, tungsten, or a compound of those, the chemical reaction was low relatively. Consequently, the ratio of the etching rate in the stirring state to that in the static state was kept at two times or so in many cases. In this case, however, since the polishing rate is also low, the ratio of the polishing rate to the etching rate in the stirring state becomes important. If such a ratio is not less than 10, the etching properties are judged to be low.
In this specification, an etching rate means a value obtained principally while the polishing agent solution is stirred so as to imitate the circumstance during polishing.
Next, description will be made for an embodiment of the present invention with reference to FIG. 5, that is, how to obtain the first and second polishing agents, both of which are substantially the same in contents and only the concentration of the BTA (Benzotriazole), used as a copper corrosion-proof agent is changed therein, although they are used for the first and second polishing processes separately. Both polishing and etching rates slow down according to an increase of the concentration of the BTA. However, how much each of those rates slows down depends on the type and concentration of the organic acid contained in the polishing agent or the concentration of the oxidizer. Generally, the effect of BTA addition to the slurry is distinct if the original slurry in which BTA is not added has strong etching properties. The effect is less distinct, on the other hand, if the original slurry has weak etching properties.
The suitable concentration of the BTA to obtain the allowable etching and polishing properties changes depending on the slurry. In FIG. 5, a relative BTA concentration of 1.0 means a concentration at which the polishing rate of a copper based alloy becomes not more than about 10% of the polishing rate when no BTA is added to the polishing agent for convenience""s sake. In FIG. 5, if the corrosive property of an original polishing agent is high, the suitable concentration may be within a relative ratio of 0.01 to 0.3. If the slurry originally has low corrosive properties, the BTA concentration may become 0.05 to above 0.7. The BTA concentration may be increased for the second polishing agent for the second step polishing more than for the first polishing agent. If such second polishing agent is used, the polishing rate for the lower metallic layer will not depend on the BTA concentration so much. This method will thus make it possible to obtain the second polishing agent that takes a faster polishing rate for the lower metallic layer and a lower polishing rate for the upper metallic layer and the insulating layer using high BTA concentration portions. The polishing rate for the lower metallic layer is changed to two or three times, however, depending on the type of the metal/compound. Preferably, therefore, a polishing agent that can contain different types organic acid as needed should be selected.
If the first polishing agent contains no abrasive powder substantially, the lower metallic layer will hardly be polished. In such a case, only a slower polishing rate of the second polishing agent is required for the insulation layer, and the second polishing agent may be any of the polishing agents either of low concentration portions or of high concentration portions shown in FIG. 5.
Classification between high corrosivity and low corrosivity for polishing agents is not universalized. In the present invention, with the respect to the first polishing agent for the upper metallic layer, low corrosivity is defined as an etching rate not more than 5 nm/min for the upper metallic layer in the static state.
Next, the processes described above will be described concretely.
(1) In the first method, the first polishing process is carried out for not only the upper metallic layer, but also for the lower metallic layer. If the upper metallic layer consists of a copper based alloy, almost all the above-described polishing agents available on the market can be used if they contain a hydrogen peroxide solution as an oxidizer. In the case of those polishing agents, the ratio of the polishing rate for the upper metallic layer to that for the lower metallic layer (tungsten, titanium, tantalum, and an alloy of those substances) is generally within xc2x1150%. Since generally, the etching rate for the upper metallic layer is fast, for example, commonly scores of nm/min, resulting in the rate ratio of the polishing rate for the upper metallic layer to the etching rate for the upper metallic layer is usually 3 to 5, the loads of the corrosion inhibitor are thus adjusted as needed, thereby controlling the etching rate so that the ratio of the polishing rate for the upper metallic layer to the etching rate for the upper metallic layer is improved to not less than 5, more preferably to not less than 10. The through-put can be improved, thereby reducing the polishing time for the upper metallic layer while watching the etching rate not so as to be reduced excessively at this time.
If a polishing agent is used for both upper and lower metallic layers, excess polishing should be avoided in the first polishing step. On the other hand, if it is misjudged that the entire polishing is finished while part of the lower metallic layer is left unpolished, a short-circuit will occur between metallic wirings, thereby deteriorating the yield of wirings patterns. This is also a disadvantage of the conventional one-step polishing method. To avoid this disadvantage, therefore, the first polishing step is finished with less excess polishing, then the second polishing is started. In this case, the second polishing agent must satisfy the condition that the polishing rate for the lower metallic layer becomes five times faster than the polishing rate for the insulating layer. This is to suppress thinning of the insulating layer in excess polishing. If the main surface of the insulator is planarized enough before polishing, the polishing rate for the lower metallic layer to that for the insulating layer should be not less than seven times, preferably not less than 10 times. On an actual wiring board, however, depressions up to around 100 nm are left as are in some cases. And, in order to remove the metallic layer from such depressions, the etching properties of the polishing agent must be improved or the insulating layer is polished away until the surface becomes planarized. If the latter method is adopted, the polishing rate for the insulating layer is set to seven to ten times faster than that for the lower metallic layer, thereby shortening the second polishing time. The relationship between the polishing rates for the insulating layer and the upper metallic layer is not so closely related, but the polishing rate for the insulating layer should preferably be not more than ⅓ of that for the upper metallic layer so as to take excess polishing into consideration.
When the lower metallic layer is to be polished, the ratio of the polishing selectivity for the upper metallic layer to that for the lower metallic layer may be adjusted according to the conditions of the first polishing process for the upper metallic layer, as well as according to planarity of the surface of the object insulating layer surface.
This completes the description for how to polish the upper and lower metallic layers. More concretely, it is premised in the above description that the upper metallic layer consists of, for example, copper or a copper based alloy and the lower metallic layer consists of, for example, titanium, tantalum, tungsten, or compounds with nitrogen and/or silicon. The lower metallic layer must not always be a single layer; it may be a laminated film consisting of two or more layers. For the lower metallic layer, tantalum, titanium, and the compounds with nitrogen will be suitable from the viewpoint of adhesive and barrier properties, but tungsten and the compound with nitrogen or nitrogen and silicon will be suitable from the viewpoint of film forming and polishing easiness.
Hereunder, description will be made for a method for materializing a polishing agent having predetermined characteristics, which enable the first method described above. The first polishing agent may be obtained by adding a very small amount of an inhibitor (protection layer forming agent) to any of the polishing agents available on the market, which contains alumina and silica abrasive powder, as well as a hydrogen peroxide solution and an organic acid as etchant chemicals. The composition of the first polishing agent is not limited only to that, of course; any other polishing agents than those available on the market may be used, and various types of organic acids and inhibitors may be added to any of those polishing agents as needed.
In the present invention, the inhibitor is selected from Benzotriazole (hereafter, to be described as BTA), BTA derivatives such as carboxylic acid, etc., dodecanethiol, Triazole, Tritriazole, and derivatives or compounds of those substances. Among those substances, the BTA is the most effective and provides a stable effect. In the present invention, it is generally confirmed that addition of an amount of 0.01 wt % to 0.05 wt % to the object polishing agent will be enough, although it depends on the type of the polishing agent. In order to avoid the reduction of the polishing rate to be caused by addition of such an inhibitor (protection layer forming agent), the second inhibitor (protection layer forming agent) is added to the polishing agent as needed. The second inhibitor (protection layer forming agent) is selected from poly-acrylic acid, poly-methacrylic acid, ammonium acid salts and compounds of those substances, or ethlenediaminetetraacetic acid. If such a substance is added to the object polishing agent, favorable low etching characteristics will be obtained even when the concentration of the first inhibitor (protection layer forming agent) is lowered. It is thus possible to avoid the reduction of the polishing rate. Although it depends on the type of the polishing agent, predetermined low etching characteristics can be obtained if the concentration of the first inhibitor contained in the first polishing agent is not less than 0.01 wt % and not more than 0.05 wt %. With those methods described above, the rate ratio of the polishing rate to the etching rate with respect to the upper metallic layer can secure 10 or over. The polishing rate for the underlying insulating layer can thus be suppressed from increasing, since the pH of the polishing solution is kept weakly acid or acid.
The required characteristics for the second polishing agent can be obtained by using, for example, a commercially available polishing agent or another type commercially available polishing agent, each of which is the same as the first polishing agent, as well as a polishing agent containing a hydrogen peroxide solution as an oxidizer and the above organic acid. The polishing rate for the lower metallic layer must be not less than 10 times faster than the etching rate for the upper metallic layer. In addition, the polishing rate for the upper metallic layer should preferably be not more than 150% of that for the lower metallic layer. The first inhibitor contained in the second polishing agent is adjusted so that its concentration becomes not less than 0.04 wt % and not more than 1 wt %. If such the first polishing agent is used also as the second polishing agent, it is only needed to increase the concentration of the first inhibitor (protection layer forming agent) to be added to the object polishing agent. In this case, the polishing rate for the upper metallic layer can be suppressed equally to or not more than the polishing rate for the lower metallic layer. Generally, the necessary characteristics will be obtained only by increasing the concentration of the first inhibitor by not less than 0.01 wt % than the first polishing agent. For example, if the concentration of the first inhibitor added to the first polishing agent is 0.04 wt %, the concentration of the first inhibitor contained in the second polishing agent must be not less than 0.05 wt %. If the concentration of the inhibitor added to the first polishing agent is 0.05 wt %, the concentration of the inhibitor contained in the second polishing agent must be 0.06 wt %. If the concentration of the inhibitor is increased, the polishing rate for the lower metallic layer is a little reduced. However, since both polishing and etching rates for the upper metallic layer are also reduced significantly at this time, the characteristics can further be improved appropriately if the improvement of the fabrication shape accuracy is focused. Since the solubility to water is low for many of inhibitors, it is difficult technically to solve a high concentration inhibitor in water. Practically, satisfactorily low etching characteristics were obtained for any polishing agent if the inhibitor was added by not less than 0.1 wt %, around 1 wt % in maximum.
The addition of the above inhibitor is intended to obtain suitable characteristics to polish the lower or upper metallic layer using the first polishing agent. The concentration of the first inhibitor contained in the first polishing agent is adjusted so as to keep the polishing rate at not more than 95% and not less than 40% of the polishing rate for the upper metallic layer when the inhibitor is not contained in the polishing agent. In this case, therefore, obtainment of the polishing characteristics takes precedence over the absolute value of the concentration. As for the second polishing agent, the polishing rate for the insulating layer could be controlled to ⅕ to {fraction (1/10)} of the polishing rate for the lower metallic layer, since the pH of the second polishing agent was weakly acid or acid. If there are depressions of not less than several nm in depth and not more than 10 xcexcm in width in an area on the surface of the insulating layer, where wiring patterns would not be made, those depressions must be eliminated by polishing this insulating layer in the second polishing process. If there is no such depression, which causes troubles in polishing, in a predetermined area, the fabrication shape of the inlaid wiring will be formed at a favorable accuracy, when the concentration of the first inhibitor contained in the second polishing agent is adjusted so as to control the polishing rate for the upper metallic layer to not more than 30% of that when the first inhibitor is not contained in the polishing agent. If the first and second polishing agents consist of the same main components and simply include a different inhibitor from each other, the polishing is done using the first polishing agent, then the second polishing agent is poured on the same polishing platen for starting the second polishing. The above method has an advantage that a mixture of those polishing agents will not affect the polishing badly. If the pH of the second polishing agent is not less than 1 and not more than 6, the lower metallic layer can be polished selectively by suppressing the polishing rate for the insulating film.
The second polishing process can also be carried out using a polishing agent that contains no abrasive powder substantially. That means the abrasive powder is inorganic and whose concentration is not more than 0.1 wt %. The polishing agent used in the first polishing process may be equivalent to that used in the above method. The second polishing agent contains an organic acid used as an etching agent, as well as an oxidizer. The first inhibitor used for the upper metallic layer is then added as needed. The organic acid should be an acid to be absorbed onto the surface of the lower metallic layer. Such an oragic acid is, for example, malonic acid, fumaric acid, malic acid, adipic acid benzoic acid (e.g., 4-cyano benzoic acid), phthalic acid, uric acid, oxalic acid, tartaric acid, lactic acid, succinic acid, citric acid. Otherwise, the second polishing agent should contain at least one of salts or compounds obtained from reaction of those acids with ammonium. In particular, malonic acid, phthalic acid, adipic acid, or each of the salts with ammonium is suitable for the treatment. The concentration of each of those substances may be not more than about 0.1 wt %. Inorganic acids such as nitric acid, sulfuric acid, phosphoric acid, etc. may be added so as to adjust the pH and the etching rate. Such a polishing agent will be able to polish away the lower metallic layer even when no inorganic abrasive powder is contained substantially.
For such a polishing agent that contains those acids, the etching rate for the first and second metallic layers is kept at not more than {fraction (1/10)} of the polishing rate only with difficulty. Consequently, the dishing to appear on both the lower and the upper metallic layers will become larger than those in the above example.
The use of such the second polishing agent will, however, be effective to reduce the polishing rate for the insulating layer far from {fraction (1/10)} of the polishing rate for the lower metallic layer. The increase of erosion to appear on the insulating layer can thus be ignored. Tungsten or an alloy including tungsten are suitable for the lower metallic layer, since the polishing rate can be set faster especially so as to be suitable for the treatment.
(2) Next, the second method will be described. In this second method, the first polishing agent hardly polishes the insulating layer and the lower metallic layer. In this case, the upper and lower metallic layers are polished completely independently of each other. The characteristics of such the first polishing agent can be obtained by adjusting the concentration of the inorganic fixed abrasive powder contained in the polishing agency to not more than 0.1 wt %, which means that no inorganic fixed abrasive powder is contained substantially. If such a polishing agent that contains no abrasive powder is used, the object metallic layer can be polished at a practical rate, while the lower metallic layer and the insulating layer can be polished at a rate of not more than {fraction (1/10)} of that for the upper metallic layer. The polishing of the upper metallic layer stops effectively when the lower metallic layer or the insulating layer is exposed. Even after the upper metallic layer is removed completely with excess polishing carried out enough, at least part of the lower metallic layer remains unpolished.
In the second polishing process for the lower metallic layer, either the first or second polishing agent described in the first method can be used as the second polishing agent.
When any of those first and second polishing agents is used, the polishing rate for the lower metallic layer must be faster than the etching rate for the upper metallic layer. Concretely, if the nominal plane of the insulating layer is planar enough, the polishing selectivity for the lower metallic layer to that for the upper metallic layer may be 3 to 5, more preferably not less than 10. The above ratio may be 0.5 to 3 or so if the surface of the insulating layer, before polishing, has depressions in various sizes of not less than 5 nm in depth, for example, scratch-like depressions of not more than 1 micron in width and 1 to 5 microns in length, as well as wide and gentle depressions of not less than 5 microns in width and not less than 10 microns in length. This is to polish the surface of the insulating layer up to the depth equal to each of those depressions. Even when the above ratio is 3 or over, the lower metallic layer can be removed only with slight excessive polishing. In this case, although the processing performance is degraded, resulting in an increase of the polishing cost, no problem will arise from the point of the fabrication accuracy.
There is also another method for using the first and second polishing agents including no inorganic abrasive powder substantially. The first polishing agent can be one of the above described polishing agents which do not contain inorganic abrasive powder substantially. The second polishing agent may be also the one for polishing the lower metallic layer, which do not include the above inorganic fixed abrasive powder substantially. The characteristics and results of polishing performed using each of the first and second polishing agents are almost the same as those in the method described above. According to this method, the polishing rate for the lower metallic layer using the second polishing agent is apt to become lower than when the polishing agent which contains inorganic abrasive powder is used. The etching rate is also apt to increase in this case. Consequently, the rate ratio of the polishing rate to the etching rate is lowered. The through-put of the polishing is also a little degraded. Those are the disadvantages of this method. On the other hand, such the second method also has the following advantages; being free of such problems as packing in the polishing agent distributor and the drain system to be caused by settling of the abrasive powder, since both first and second polishing agents contain no inorganic abrasive powder substantially, as well as becoming easier in waste treatment. The method can thus reduce the cost of the whole polishing process significantly. Since a polishing agent that contains no abrasive powder substantially hardly polishes the insulating layer, such a polishing agent can have an advantage for reducing erosion.
In addition, the well-known endpoint monitoring method can be used to detect the end point of the polishing of each metallic layer. For example, the 1996 Proceedings First International Chemical Mechanical Polish (pp.256-262) describes an optical method for detecting end points of polishing. Concretely, since the reflectivity of light differs between lower and upper metallic layers, changes of the reflected light strength can be detected when the lower metallic layer is exposed. If a polishing process proceeds, thereby the lower metallic layer is polished away, so that the underlying insulating layer is exposed, then the change of the reflected light intensity is changed more remarkably. There is also a method for detecting such end points of polishing by detecting changes of a torque for rotating the polishing platen and the wiring board carrier during polishing. If the object polishing agent contains no abrasive powder, very small no mechanical friction due to abrasive powder occurs, and the torque change due to film layer change can be detected very easily.
When in polishing of the upper or lower metallic layer, generally, a polishing rate distribution within about xc2x110% appears within a wiring board. Consequently, part of the metallic layers will be left unpolished in some cases even when an end point detector is used to judge end points of polishing. In such a case, it is only needed to repeat polishing of each metallic layer properly using the first and second polishing agents.
Of course, a different polishing platen should preferably be used for each of the first and second polishing agents, since the characteristics differ between those agents as described above. In particular when one of the first and second polishing agents contains no abrasive powder substantially and the other contains abrasive powder, a different polishing platen should be used for each of them so as to avoid the interaction between them. It is also possible for them to share a polishing platen, of course. Even in such a case, the second polishing process will not be affected by the first polishing process so much, since the concentration of the first inhibitor for the upper metallic layer is higher in the second polishing agent than in the first polishing agent. If a plurality of wiring boards are to be treated continuously, however, a high concentration of the inhibitor will cause the polishing of the upper metallic layer unstable. To avoid such a problem, therefore, a measure should be taken to lower the concentration of the inhibitor beforehand. The measure may be, for example, pouring pure water and a solution including a low concentration inhibitor on the surface of the polishing pad, as well as removing the abrasive powder and the inhibitor (protection layer forming agent) from the surface of the polishing pad using a diamond plate, a brush, etc. Although the present invention uses the upper metallic layer consisting of a copper based alloy, the same polishing conditions may also apply to an aluminum based alloy, which reacts with chemicals strongly. Also in this case, the same favorable result will be assured.
The methods described above are all for polishing object layers using a polishing agent that contains inorganic abrasive powder or a polishing agent that contains no inorganic abrasive powder substantially while the wiring board, pressed against the polishing pad, is moved. On the contrary, there is also another applicable method, which is an intermediate one between those two methods. The method uses a sheet including inorganic fixed abrasive powder on the polishing pad. Ina broad sense, such a sheet is included in the resin bonded whetstone. The sheet is placed on the surface of a rotating platen, which is pressed against the object wiring board for polishing. The whetstone consists of silica, alumina, or cerium dioxide powder. The polishing agent is a solution containing an oxidizer and an organic acid, as well as an inhibitor as needed. This solution may or may not include inorganic abrasive powder. While in polishing, abrasive powder is generated from the whetstone and this makes it more effective to polish the object wiring board. Since such a whetstone is harder substantially than a polishing pad consisting of only resin, the planarization effect is more improved, thereby reducing occurrence of the dishing phenomenon effectively.
However, since the abrasive powder is fixed in the object whetstone, scratches are apt to be created on the surface of the wiring board. To avoid this, therefore, the whetstone should be given characteristics of a higher voiding ratio and easiness to be removed.
According to the present invention, wirings can be inlaid accurately without leaving any portion of the lower metallic layer unpolished and by reducing occurrence of dishing on the upper metallic layer and erosion on the insulating layer, respectively. In particular when the surface of the insulating layer is already planar enough before polishing, the first inhibitor of a satisfactory high concentration is added to the second polishing agent, so that the upper metallic layer is hardly polished. In this case, the polishing force in the second polishing process eliminates the distribution of groove patterns almost completely, thereby both dishing and erosion can be prevented from occurrence all over the predetermined area of the object wiring board. At this time, the depth of dishing and erosion becomes not more than ⅕ of those to appear at a wiring board formed using the related art technique.